Barr's announcement highlights the state of American politics these days. The major parties seem owned and operated by the 1% – the wealthy and powerful interests that have all but taken control of Congress and state governments in the past decade. Minor parties like the Greens are hobbled by laws, written by major parties, that make serious challenges at high levels next to impossible; and their candidates tend either to be unknown or implausible in other ways. This conundrum is less the case in some other countries, where the Greens, enabled in part by laws friendlier to greater political diversity, have won seats in national legislatures.

America's Greens are best-known for their presidential candidates in recent years, most notably Ralph Nader in 2000. He had a long record of activism on behalf of ordinary people and took earnest positions, but refused to give way when it became clear that his candidacy was helping George W Bush into the White House. His and his supporters' insistence that there was no serious difference between Bush and Al Gore, absurd on its face (Exhibit A: the US supreme court), did the Greens no favors. Later, the 2008 Green presidential candidate, Cynthia McKinney, had credentials as a former member of Congress, but her positions – including both 9/11 and Tupac "truther" overtones – were considered extreme. But no one paying attention could doubt the real differences, at least in their campaign promises, between Barack Obama and John McCain.

Obama, of course, broke many of his promises. After mostly doing Wall Street's bidding for three years and putting climate change on the backburner, he has in recent weeks paid more liberal-leaning attention to the issues that motivate protesters and environmentalists; in the latter case, his administration blocked, for the time being, the Keystone XL pipeline that was to bring tar sands oil from Canada to the US Gulf coast. On most issues, however, Obama has governed from the political right, including assertions of executive power – including a penchant for secrecy and antipathy to civil liberties – that are even more extreme than Bush's. The Republicans, meanwhile, have moved much further to the right. They flaunt their love for the 1% and contempt for the environment (when it gets in the way of commerce, at any rate).

In such circumstances, one might imagine an opening for a third party that, among other things, believed in civil liberties; advocated genuine reform of the corporatism that now rules the economy and government; and pushed for an energy policy that sharply reduced America's carbon footprint. The Greens' platform endorses all three. But are their candidates plausible?

Certainly Barr, while by no means a dummy, is not. Even she agrees: if we are to believe this tweet, she expects Jill Stein to win the nomination at the party's July convention in Baltimore. In other words, her campaign is a statement and not much more.

Stein is plainly a serious person with well-considered positions. A doctor who graduated with honors from Harvard and Harvard medical school, she has run several times for statewide office there, and won a significant number of votes in a 2006 campaign for Massachusetts secretary of state. But she hasn't won any elections outside of her home town of Lexington, and few people could plausibly suggest that she has anything remotely near the experience needed to be president.

If she is nominated, Stein will perform a useful service: reminding the public – provided the political press bothers to pay any attention (not a given) – that Obama has broken many promises that the Republicans would never make in the first place. But the Greens would have more influence if they could recruit someone with more obvious gravitas.

More important, though, the Greens and other minor parties have not done enough of the hard work it takes to become power players in America's political system. The deck is stacked against them, yes, but they could all be doing more to find and push candidates at the local level. They have won some races, to be sure. But only when they start to gain orders-of-magnitude more seats on school boards, town and county governments and state legislatures, will they be a real force.